Earth fill reinforcement systems can be used, for example, to ensure stability and to minimize the lateral movement of a facing wall constructed to retain a volume of earth fill or rock material which is backfilled behind the wall. Such walls are used typically for separating regions of ground at two different levels.
The backfill material can be stabilized using lengths of strong, flat webbing or other flexible strip material, laid on successive layers of backfill material as the space behind the retaining wall facing is filled. The strips are attached to the rear side of the retaining wall and then tensioned away from the wall and pegged to the ground, such that when the next layer of backfill material is added, the strips are held in position, and the cumulative weight of the earth backfill on the strips produces sufficient friction between strips and earth to reinforce the backfill and hold the retaining wall in place.
Retaining or facing walls may be constructed from pre-cast concrete facing panels, for example, with each facing panel having several attachment points on its rear surface. These attachment points can be designed as loop channels, so that a retaining strip can be fed into one opening in the rear surface of the panel, through a channel inside the panel, and out through a second opening in the rear surface of the panel. Where loop-channel anchorages are used, the pre-cast panels are cast using void-formers so that the loop channels are formed as part of the casting process. Each loop channel is formed such that a flat, broad strip can be fed through the channel and around a part of the panel volume which will be termed the “core” of the channel in this application. This term is thus used to refer to that part of the volume of the panel around which the strip will pass when it is fed through the loop channel. It is the core which will bear the pulling force acting between the panel and the strip once the wall and the backfilling is complete. The core is typically a contiguous part of the panel material (reinforced concrete, for example, with some reinforcement passing through the core element), although the core may also be constructed from a different material (for example a cylinder of steel or carbon fibre, or high density concrete, which may be cast into the concrete of the body of the panel).
In order to provide a strong anchoring point, the loop-through channels should be cast as deep as possible into the volume of the panel, with a core which is also as deep as possible. A further consideration is that the two openings of the channel should be as close together as possible, in order that the strips emerging from the openings are not subjected to excessive local tension by the weight of the backfill material as it settles. Similarly, the load-bearing inner surface of the core element should have as large a radius of curvature as practicable, and have an even surface free of projections or discontinuities, in order to minimize the amount of localized stress on that part of the strip which is in contact with the core once the strip is under tension.
United States patent application U.S. Pat. No. 5,839,855 describes a pre-cast facing panel with loop-channels cast into its rear surface. The channels are formed by using plastic shell mould formers which are cast into the concrete and remain in the concrete. Cast-in mouldings must be in stock and available at the time of casting, and they represent a significant extra manufacturing cost.
The former arrangement described in U.S. Pat. No. 5,839,855 comprises two former-halves which are rotated about a common hinge which is located outside of the volume of the panel being cast. The inner and outer surfaces of each half-former have a constant radius of curvature over the inner region of the channel. In order for the void former to be removable without interference with the concrete, the hinge axis is located between the centre of curvature of the outer (forward-most) surface of the anchorage channel and the centre of curvature of the inner (rear-most) surface of the anchorage channel, with the result that the formed shape of the channel is limited to wide and/or shallow channel geometries. In particular, the restriction on the geometry of the inner surface of the half-formers means that there is an undesirable trade-off between the channel depth and the distance between the openings. The hinged construction also means that the former is necessarily bulky and heavy. The hinge axis must inevitably lie outside the concrete.